1. Field of the Invention
The present invention relates to ink pens for ink-jet printers, and more particularly, to an apparatus for controlling the pressure within the reservoir of an ink pen.
2. Description of Related Art
Ink-jet printers have become established as reliable and efficient printing devices. Typically, an ink-jet printer, utilizes a print head which is moved relative to a printing surface. A control system activates the moving print head at the appropriate locations causing the print head to eject, or jet, ink drops onto the printing surface to form desired images and characters. Such printers typically include an ink pen which serves as a reservoir for storing ink and provides a means of supplying ink, as needed, to the print head.
There are two commonly used systems for ejecting ink from a print head. The first is a thermal bubble system and the second is a piezoelectric system. A print head using either system typically includes a plurality of orifices, each orifice having an associated chamber. In operation, ink is supplied via an inlet to the chamber. Upon activation, the ink is forced, or jetted, from the chamber through the orifice and onto the printing surface. In thermal bubble type print heads, the ink in the chamber is heated or vaporized, typically by a thin film resistor. The rapid expansion which results from vaporization of the ink forces a quantity of ink from the chamber through the orifice. In piezoelectric type print heads, a piezoelectric element creates a pressure wave within the chamber which ejects a quantity of ink through the orifice.
Although both thermal bubble and piezoelectric print heads provide a reliable and efficient means of jetting ink from an orifice, both types of print heads generally have no mechanism to prevent the free flow of ink through the orifice when the print head is not activated. If this occurs, ink may leak, or drool, uncontrollably onto the printing surface to produce an undesirable ink spot. In addition, leaking ink may build up on the print head and impair the proper operation of the print head.
To alleviate these problems, many ink-jet printers supply ink from the ink pen to the print head at a slight underpressure or back pressure. As used herein a positive back pressure is used to refer to a pressure within an ink pen that is lower than the ambient pressure surrounding the print head orifice.
To be effective, the back pressure must be maintained within a desired operating range. That is, the back pressure must be large enough to prevent the unwanted free flow of ink through the orifice. At the same time, the back pressure must be small enough that the print head, when activated, can overcome the back pressure and eject the ink in a consistent and predictable manner. To meet these constraints and provide optimum operation of the ink-jet printer, a fairly constant and predictable back pressure should be maintained.
The back pressure of an ink pen is affected by changes in either the ambient pressure or the internal pressure. For example, if an ink pen is subject to an increase in altitude, such as during transport aboard an aircraft, the ambient pressure may decrease substantially. Unless the back pressure of the ink pen increases accordingly, the ambient pressure level may drop below that of the back pressure and ink will likely leak from the print head. In addition, as ink is depleted from the ink pen reservoir the back pressure within the ink pen will tend to increase. Without some mechanism for compensating for this, the back pressure may exceed the operating range of the print head and the ink pen will become inoperative. Temperature variations may cause the ink and air within the ink pen to contract or expand, thereby affecting the back pressure. All of these factors must be accounted for in order to ensure consistent trouble-free operation of the ink-jet printer.
One type of ink pen uses a variable volume reservoir to solve these problems. For example, the reservoir may be of a flexible material which can expand or contract. Alternatively, the reservoir may have sleeve and piston configuration or utilize an expandable bladder as an internal accumulator. In this type of ink pen, as the volume of ink within the reservoir varies due to depletion, thermal variations, or the like, the volume of the reservoir also varies. Although a significant improvement over previous ink pens, such devices suffer from certain drawbacks.
For example, such devices do not necessarily provide a constant back pressure. Rather, a reservoir with a freely variable volume will tend to maintain an internal reservoir pressure which is equal to the ambient pressure, that is, zero back pressure. To overcome this problem, many variable volume reservoirs use a resilient member, such as a spring, to constantly urge the reservoir toward an increased volume. In this manner, the desired back pressure is created.
Because variable volume reservoirs inherently have maximum and minimum limitations on the size of the reservoir, they are typically least effective when the ink pen is either nearly full or nearly empty. For example, if a new ink pen with a variable volume reservoir is filled to capacity with ink, the reservoir is unable to further expand in response to back pressure changes. As a result, if the fluid volume within the reservoir expands due to a change in back pressure, a quantity of ink may be forced out through the print head. To compensate for this many new pens are not completely filled with ink. Even more significant, variable volume reservoirs typically have a minimum volume which is greater than zero. As the pen nears depletion and the reservoir shrinks to this minimum volume, further ink depletion raises the back pressure above the operating range of the print head. As a result, a quantity of unusable ink will remain in each discarded pen.
To reduce this problem, some ink pens incorporate a "bubble generator." A bubble generator is an orifice formed in the ink reservoir of an ink pen to allow fluid communication between the interior of the reservoir and the ambient atmosphere. The orifice is sized such that the capillarity of the ink normally retains a small quantity of ink in the orifice as a liquid seal. The geometry of the orifice is such that when the back pressure approaches the limit of the operating range of the print head the back pressure overcomes the capillarity of the ink and the liquid seal is broken. Ambient air then "bubbles" into the reservoir to reduce the back pressure. Ideally, when the back pressure drops, ink from the reservoir reenters the orifice and reinstates the liquid seal.
However, if the seal breaks and the orifice is not submerged, there is no ink to reinstate the seal and the back pressure may be lost. In addition, if the ink level drops or the pen is oriented in such a manner that the orifice is above the ink level within the reservoir, the liquid seal may weaken and fail over time. This would permit the free flow of ambient air into the reservoir, eliminate the back pressure, and allow the ink pen to drool.